Heat oil in a Dutch oven over medium heat. Add onion, celery and carrot and cook, stirring occasionally, until softened, about 5 minutes. Add water, broth, barley, black beans, great northern beans, kidney beans, chili powder, cumin and oregano. Bring to a lively simmer over high heat. Reduce heat to maintain a simmer and cook, stirring occasionally, until the beans are tender, 1¾ to 2½ hours (adding more water, ½ cup at a time, if necessary or desired). Season with salt.